Existing X-ray analysers for bulk materials typically have detection limits of the order of a few tens of parts-per-million or larger, making such analysers unsuited to the measurement of valuable trace elements such as gold and platinum. Typically, valuable trace elements are mined at concentrations of a few parts-per-million or less and existing analysers are prone to interference due to scattered radiation or fluorescence emission from elements other than the element of interest.
X-ray focusing based on Bragg reflection at curved crystals allows collection of incident radiation over large solid angles and subsequent monochromatisation. By means of an appropriate curvature of the crystalline structure, efficient X-ray focusing becomes possible. By an appropriate variation of the Bragg reflection angle, it is possible to enhance the radiation in an energetic region of interest with respect to energetically undesirable parts of the spectrum.
Advances in Bragg diffraction materials offer significant potential as they enable analysers to combine selectivity with large effective solid angles. Highly Oriented Pyrolytic Graphite (HOPG)1 is of particular interest and has been shown to be a very efficient X-ray monochromator. Optical elements having a boundary surface in the form of a curved surface are manufactured where the curved surface is formed from HOPG crystals. Perhaps the most distinguishing feature of HOPG crystals is a large mosaicity which leads to the largest integral reflectivity among all known crystals. Another feature of HOPG is that it can be formed into various shapes including an ellipse, a logarithmic spiral, a Johann geometry with singly bent (cylindrical) or doubly bent (spherical or toroidal) HOPG, or a Johansson geometry. 1 Also referred to occasionally as highly annealed pyrolytic graphite (HAPG)
FR 2579752 describes the use of an analyser incorporating dual logarithmic spiral focusing elements. The disclosure reports detection limits of 0.05-0.1 parts-per-million for elements in solution, which would be equivalent to approximately 0.8-2.0 parts-per-million for elements in a slurry. However, such an analyser is unsuited to the measurement of valuable trace elements such as gold and platinum.
It is desired to develop an analyser which is capable of yielding improvements in its detection limit compared to the results reported previously.